Hormones, such as estrogen or prolactin, affect cerebral neurotransmitter receptors. Estrogen increases the following: (1) the density of striatal dopamine (DA) receptors; (2) stereotypy induced by DA agonists; (3) catalepsy produced by a DA antagonist; and (4) DA agonist-induced rotation in unilaterally lesioned rats. The increase in DA receptor density is specific and restricted to one population. In the striatum, this increase is prevented by hypophysectomy, suggesting involvement of a pituitary factor. The increase in DA receptor density after chronic haloperidol is also attenuated by hypophysectomy, again suggesting a pituitary factor, such as prolactin, which by itself increases the density of striatal DA receptors. We have also analyzed neurochemical codes of cataleptic states, behaviorally similar to opiate and neuroleptic catalepsy, and their experimental relationship to other psychotropic-drug-induced behaviors, including stereotypy, locomotor hyperactivity, distinct epileptic seizures, coma, and death. Other experiments show that contrasting reflex mechanisms (i.e., vestibular controls) underlie opiate vs. neuroleptic catalepsy. These findings may be relevant to neurologic, psychiatric, neuroendocrinologic disorders, and drug-induced side-effects.